The Mixed Lineage Leukemia protein-1 (MLL1) catalyzes histone H3 lysine 4 (H3K4) methylation, which is an epigenetic mark essential for the regulation of HOX genes in hematopoiesis and development. Translocations that disrupt the MLL1 gene are present in a unique group of acute leukemias, often predicting a poor prognosis. Other MLL1 rearrangements and amplifications increase MLL1's enzymatic activity and are oncogenic. MLL1 contains an evolutionarily conserved ~130 amino acid SET domain that catalyzes H3K4 methylation. Recent studies indicate that the enzymatic activity of MLL1 is regulated by a conserved complex of proteins including WDR5, RbBP5, and ASH2L. These proteins form an independent complex that binds to MLL1 and regulates its ability to mono-, di-, or trimethylate H3K4 substrates, a phenomenon known as 'Product Specificity'. Since different levels of methylation of H3K4 are associated with different transcriptional outcomes, it is imperative to understand the molecular mechanisms by which the product specificity of MLL1 is regulated. Despite the important biological role of MLL1 and its involvement in human leukemia, there is currently little information about the protein-structural features that are responsible for the enzymatic activity of the MLL1 core complex. The long-term goal of this research is to fully characterize the mechanisms for the regulation of H3K4 methylation by the MLL1 core complex. This proposal takes a structure-function approach to investigate the molecular mechanisms for the enzymatic activity of the MLL1 core complex. We have obtained novel preliminary results indicating that the intrinsic product specificity of the MLL1 SET domain is that of a slow monomethyltransferase, but becomes a fast dimethyltransferase when in complex with WDR5, RbBP5, and Ash2L. Here we propose to determine the mechanisms responsible for H3K4 dimethylation activity of the MLL1 core complex. In addition, we will determine the protein-structural features that account for its assembly. To address these aims, we combine molecular, biochemical, and biophysical approaches to investigate the regulation of H3K4 methylation by the MLL1 core complex. This information will increase our understanding of a key enzyme complex and how it is regulated in the pathways that control transcriptional activation in eukaryotes. This investigation is important because it may lead to better diagnostics and the rational design of anti-cancer drugs that inhibit MLL1's enzymatic activity.